Tobacco (Nicotiana tabacum L.) is an important commercial crop in the United States as well as in other countries. In tobacco plants, N-demethylation of nicotine results in nornicotine, a secondary alkaloid known to be a precursor for formation of N-Nitrosonornicotine (“NNN”) in cured leaves. NNN is an undesired component of cured leaves.
The predominant alkaloid found in commercial tobacco varieties is nicotine, typically accounting for 90-95% of the total alkaloid pool. The remaining alkaloid fraction is comprised primarily of three additional pyridine alkaloids: nornicotine, anabasine, and anatabine. Nornicotine is generated directly from nicotine through the activity of the enzyme nicotine N-demethylase. Nornicotine usually represents less than 5% of the total pyridine alkaloid pool, but through a process termed “conversion,” tobacco plants that initially produce very low amounts of nornicotine give rise to progeny that metabolically “convert” a large percentage of leaf nicotine to nornicotine. In tobacco plants that have genetically converted (termed “converters”), the great majority of nornicotine production occurs during the senescence and curing of the mature leaf (Wernsman and Matzinger (1968) Tob. Sci. 12:226-228). Burley tobaccos are particularly prone to genetic conversion, with rates as high as 20% per generation observed in some cultivars.
During the curing and processing of the tobacco leaf, a portion of the nornicotine is metabolized to the compound NNN, a tobacco-specific nitrosamine (TSNA) that has been asserted to be carcinogenic in laboratory animals (Hecht and Hoffmann (1990) Cancer Surveys 8:273-294; Hoffmann et al. (1994) J. Toxicol. Environ. Health 41:1-52; Hecht (1998) Chem. Res. Toxicol. 11:559-603). In flue-cured tobaccos, TSNAs are found to be predominantly formed through the reaction of alkaloids with the minute amounts of nitrogen oxides present in combustion gases formed by the direct-fired heating systems found in traditional curing barns (Peele and Gentry (1999) “Formation of Tobacco-specific Nitrosamines in Flue-cured Tobacco,” CORESTA Meeting, Agro-Phyto Groups, Suzhou, China). Retrofitting these curing barns with heat-exchangers virtually eliminated the mixing of combustion gases with the curing air and dramatically reduced the formation of TSNAs in tobaccos cured in this manner (Boyette and Hamm (2001) Rec. Adv. Tob, Sci. 27:17-22.). In contrast, in the air-cured Burley tobaccos, TSNA formation proceeds primarily through reaction of tobacco alkaloids with nitrite, a process catalyzed by leaf-borne microbes (Bush et al. (2001) Rec. Adv. Tob. Sci, 27:23-46). Thus far, attempts to reduce TSNAs through modification of curing conditions while maintaining acceptable quality standards have not proven to be successful for the air-cured tobaccos.